Multicast signal transmission power control method and base station using the same

ABSTRACT

In a transmission power control method of the present invention, a value of a received signal quality parameter of a received multicast signal is measured. A parameter signal, indicating the received signal quality parameter value, is transmitted from a plurality of mobile stations to a base station through a radio link. The parameter signals from the mobile stations are received at the base station through the radio link. A power control value of each of the mobile stations is determined based on the received signal quality parameter values of the received parameter signals. The transmission power of the multicast signal, sent to each of the mobile stations, is controlled based on the determined power control value.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a multicast signal transmissionpower control method that controls a transmission power of a multicastsignal that is transmitted by a base station to a plurality of mobilestations through a radio link. Further, the present invention relates toa base station that uses the transmission power control method for thetransmission of the multicast signal.

[0003] 2. Description of the Related Art

[0004] Transmission of a multicast signal across a mobile radio link isknown. In such multicast communications, a multicast signal issimultaneously transmitted from a base station to each of a plurality ofspecified mobile stations. Such multicast signals, sent by the basestation, contain identical messages and different destinations, and theidentical message is delivered to each of the mobile stations specifiedas the destination stations that receive it.

[0005]FIG. 8 shows a conventional multicast signal communication system.In the communication system shown in FIG. 8, a base station 1 isprovided, and this base station 1 transmits a multicast signal 4. Aplurality of mobile stations 5 are provided in the communication system,and each mobile station 5 receives the multicast signal 4 from the basestation 1. For the sake of simplicity of description, only one mobilestation 5 is shown in FIG. 8, and the remaining mobile stations 5, whichreceive the multicast signal 4, are omitted.

[0006] In the base station 1, the original message of the multicastsignal 4 is supplied from an input terminal 2, and it is input to atransmitter (TX) 3. In the transmitter 3, the carrier wave is modulatedin accordance with the original message from the input terminal 2, andthe multicast signal 4 is produced. After the modulation is performed,the transmitter 3 transmits the multicast signal 4 to one of the mobilestations 5 based on a predetermined transmission power.

[0007] In the mobile station 5 (or one of the plurality of mobilestations 5), the multicast signal 4 from the base station 1 is receivedat a receiver (RX) 6. After the demodulation of the received multicastsignal is performed, the receiver 6 supplies the demodulated multicastsignal to an output terminal 7, and it is output from the outputterminal 7 to internal units of the mobile station 5.

[0008] In the communication system of FIG. 8, the base station 1 is notprovided with a mechanism that controls the transmission power of thebase station 1 used to transmit the multicast signal 4 to the mobilestation 5, such that the transmission power is suited to the receivedsignal quality (or the grade of service) of the mobile station 5. It isdifficult for the base station 1 to effectively reduce the transmissionpower of the base station to the optimum level for transmitting themulticast signal to the mobile stations so as to be in conformity withthe received signal quality required for the mobile stations whichreceive the multicast signal.

[0009] In order to ensure that the received multicast signal conforms tothe received signal quality of all the mobile stations located with therelated cell of the base station 1, it is necessary that thepredetermined transmission power, used by the base station 1 of theconventional communication system, be set to a level which is largerthan the marginal transmission power needed to transmit the multicastsignal from the base station 1 to the mobile stations. The base station1 must always use the predetermined transmission power that is largerthan the smallest possible transmission power, because the base station1 includes no transmission power control mechanism.

[0010] However, when the mobile stations densely exist at locations nearthe base station 1, the predetermined transmission power of the basestation 1 becomes excessively large. This will cause the interference ofthe transmission power of the base station 1 for the receiver mobilestations which receive the multicast signal with that for thenon-receiver mobile stations which do not receive the multicast signal.It is difficult for the conventional multicast signal communicationsystem to provide efficient use of the transmission power of the basestation for the transmission of the multicast signal to the mobilestations.

SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide an improvedmulticast signal transmission power control method in which theabove-described problems are eliminated.

[0012] Another object of the present invention is to provide a multicastsignal transmission power control method that can effectively reduce thetransmission power of the base station to transmit the multicast signalto the mobile stations without changing the received signal quality ofthe mobile stations which receive the multicast signal and withoutcausing the interference of the transmission power of the base stationto the mobile stations which do not receive the multicast signal.

[0013] Another object of the present invention is to provide a basestation that can effectively reduce the transmission power of the basestation to transmit the multicast signal to the mobile stations withoutchanging the received signal quality of the mobile stations whichreceive the multicast signal and without causing the interference of thetransmission power of the base station to the mobile stations which donot receive the multicast signal.

[0014] The above-mentioned objects of the present invention are achievedby a method of controlling a transmission power of a multicast signalthat is transmitted from a base station to a plurality of mobilestations through a radio link, the method comprising the steps of:measuring a value of a received signal quality parameter of a multicastsignal received at the mobile stations; transmitting a parameter signal,indicating the received signal quality parameter value, from the mobilestations to the base station through the radio link; receiving theparameter signals from the mobile stations at the base station throughthe radio link; determining a power control value of each of the mobilestations based on the received signal quality parameter values of theparameter signals received at the base station; and controlling thetransmission power of the multicast signal, sent to each of the mobilestations, based on the determined power control value.

[0015] The above-mentioned objects of the present invention are achievedby a base station which controls a transmission power of a multicastsignal that is transmitted to a plurality of mobile stations through aradio link, the base station comprising: a receiver which receivesparameter signals from the mobile stations through the radio link, eachparameter signal indicating a value of a received signal qualityparameter of the multicast signal received at one of the mobile station;a determination unit which determines a power control value of each ofthe mobile stations based on the received signal quality parametervalues of the parameter signals received by the receiver; and atransmission power controller which controls the transmission power ofthe multicast signal, sent to each of the mobile stations, based on thedetermined power control value.

[0016] The above-mentioned objects of the present invention are achievedby a method of controlling a transmission power of a multicast signalthat is transmitted from a base station to a plurality of mobilestations through a radio link, the comprising the steps of: transmittingthe multicast signal to the mobile stations through the radio link;receiving an automatic repeat request ARQ signal from each of the mobilestations at the base station through the radio link, wherein each of themobile stations transmits the ARQ signal to the base station when anerror in demodulation of a received multicast signal occurs; detectingwhether at least one of a plurality of ARQ signals from the mobilestations is received at the base station; outputting a power controlsignal indicating a result of the ARQ-signal detection; and controllingthe transmission power of the multicast signal, sent to each of themobile stations, based on the ARQ-signal detection result indicated bythe power control signal.

[0017] The above-mentioned objects of the present invention are achievedby a base station which controls a transmission power of a multicastsignal that is transmitted to a plurality of mobile stations through aradio link, the base station comprising: a transmitter which transmitsthe multicast signal to the mobile stations through the radio link; areceiver which receives an automatic repeat request ARQ signal from eachof the mobile stations through the radio link, wherein each of themobile stations transmits the ARQ signal to the base station when anerror in demodulation of a received multicast signal occurs; a signalcounter unit which detects whether at least one of a plurality of ARQsignals from the mobile stations is received by the receiver, andoutputs a power control signal indicating a result of the ARQ-signaldetection; and a transmission power controller which controls thetransmission power of the multicast signal, sent to each of the mobilestations, based on the ARQ-signal detection result indicated by thepower control signal of the signal counter unit.

[0018] In the multicast signal transmission power control method and thebase station of the present invention, the parameter signals from themobile stations are received at the base station through the radio link,each parameter signal indicating a value of the received signal qualityparameter of the received multicast signal. A power control value ofeach of the mobile stations is determined based on the received signalquality parameter values of the received parameter signals. Thetransmission power of the multicast signal, sent to each of the mobilestations, is controlled based on the determined power control value. Themulticast signal transmission power control method of the presentinvention is effective in reducing the transmission power of the basestation to transmit the multicast signal to the mobile stations withoutchanging the received signal quality of the mobile stations whichreceive the multicast signal and without causing the interference of thetransmission power of the base station to the mobile stations which donot receive the multicast signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Other objects, features and advantages of the present inventionwill become apparent from the following detailed description when readin conjunction with the accompanying drawings.

[0020]FIG. 1 is a block diagram of a first preferred embodiment of themulticast signal transmission system of the invention.

[0021]FIG. 2 is a block diagram of a second preferred embodiment of themulticast signal transmission system of the invention.

[0022]FIG. 3A, FIG. 3B and FIG. 3C are diagrams for explainingoperations of the multicast signal transmission system of the secondpreferred embodiment.

[0023]FIG. 4 is a block diagram of a third preferred embodiment of themulticast signal transmission system of the invention.

[0024]FIG. 5 is a block diagram of a fourth preferred embodiment of themulticast signal transmission system of the invention.

[0025]FIG. 6 is a timing chart for explaining operations of themulticast signal transmission system of the fourth preferred embodiment.

[0026]FIG. 7 is a block diagram of a fifth preferred embodiment of themulticast signal transmission system of the invention.

[0027]FIG. 8 is a block diagram of a conventional multicast signalcommunication system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0028] A description will now be provided of preferred embodiments ofthe present invention with reference to the accompanying drawings.

[0029]FIG. 1 shows a first preferred embodiment of a multicast signaltransmission system in which the transmission power control method ofthe invention is embodied.

[0030] In FIG. 1, the elements that are essentially the same ascorresponding elements in FIG. 8 are designated by the same referencenumerals, and a description thereof will be omitted.

[0031] According to the transmission power control method of theinvention, each of a plurality of mobile stations (MS) measures a valueof a received signal quality (or the grade of service) parameter of areceived multicast signal, which is received from a base station (BS)via a radio link. Suppose that a specific one of the grades of serviceis assigned, in advance, for each of the plurality of mobile stations.

[0032] In the present embodiment, a reception power of a receivedmulticast signal is measured by each of the mobile stations as being thereceived signal quality parameter value that is the base to determine apower control value of each mobile station, and a transmission power ofthe multicast signal of the base station with respect to each mobilestation is controlled based on the power control value.

[0033] As shown in FIG. 1, in the multicast signal transmission systemof the present embodiment, a base station 11 is provided, and the basestation 11 transmits a multicast signal 4 to a plurality of mobilestations 21 through a mobile radio link. The plurality of mobilestations 21 are provided in the multicast signal transmission system,and each mobile station 21 receives the multicast signal 4 from the basestation 11.

[0034] For the sake of simplicity of description, only one mobilestation 21 is shown in FIG. 1, and the remaining mobile stations 21,which receive the multicast signal 4 from the base station 11, areomitted.

[0035] In the base station 11 shown in FIG. 1, a receiver (RX) 12, atransmission power determination unit (T/P DETERMIN) 16, the transmitter(TX) 3, and a transmission power controller 17 are provided. In thedetermination unit 16, a memory 13, a comparator 14, and a referencereception power value (Cref) 15 are provided. As described later, thetransmission power controller 17 controls a transmission power of themulticast signal 4, sent to each of the mobile stations 21, based on apower control value indicated by an output signal of the determinationunit 16.

[0036] In the base station 11, an original message of the multicastsignal 4 is supplied from the input terminal 2, and it is input to thetransmitter (TX) 3. In the transmitter 3, the carrier wave is modulatedin accordance with the original message from the input terminal 2, sothat the multicast signal 4 is produced. In the transmission powercontroller 17, the transmission power of the multicast signal 4, sent toeach of the mobile stations 21, is controlled based on the power controlvalue indicated by the output signal of the determination unit 16. Afterthe modulation is performed, the transmitter 3 transmits the multicastsignal 4 to each of the mobile stations 21 through the network, based onthe controlled transmission power of the transmission power controller17.

[0037] In the mobile station 21, the receiver (RX) 6, a received signalquality measurement unit (R/Q MEASURE) 22 and a transmitter (TX) 23 areprovided. In the present embodiment, the measurement unit 22 is a powermeasuring circuit that measures a reception power of the multicastsignal 4 received at the mobile station 21. The measurement unit 22supplies a detection signal, indicating the reception power of themulticast signal 4 as the received signal quality parameter value, tothe transmitter 23. In the transmitter 23, the carrier wave is modulatedin accordance with the reception power, and the transmitter 23 transmitsa parameter signal, indicating the reception power as the receivedsignal quality parameter value, to the base station 11 through an uplink32 of the radio link.

[0038] Similar to the communication system of FIG. 8, in the mobilestation 21 shown in FIG. 1, the multicast signal 4 from the base station11 is received at the receiver (RX) 6. After the demodulation of thereceived multicast signal is performed, the receiver 6 supplies thedemodulated multicast signal to the output terminal 7, and it issupplied from the output terminal 7 to internal units of the mobilestation 21.

[0039] In the multicast signal transmission system of FIG. 1, themeasurement unit 22 measures a reception power of the multicast signal 4received at the mobile station 21. More specifically, the measurementunit 22 measures the reception power of the received multicast signal 4at a suitable timing that is suited to the signal structure of themulticast signal 4. For example, if the multicast signal 4 is providedin the form of slots, the detection of the reception power is performedby the measurement unit 22 at the suitable timing on a slot basis. Ifthe multicast signal 4 is provided in the form of blocks, the detectionof the reception power is performed by the measurement unit 22 at thesuitable timing on a block basis. If the multicast signal 4 is providedin the form of frames, the detection of the reception power is performedby the measurement unit 22 at the suitable timing on a frame basis.

[0040] The measurement unit 22 supplies the detection signal, indicatingthe reception power of the multicast signal 4 as the received signalquality parameter value, to the transmitter 23. In the transmitter 23,the carrier wave is modulated in accordance with the reception power,and the transmitter 23 transmits the parameter signal, indicating thereception power as the received signal quality parameter value, to thebase station 11 through the uplink 32 of the radio link. The uplink 32may be a channel of the radio link used for random access.

[0041] In the base station 11 of the multicast signal transmissionsystem of FIG. 1, the receiver 12 receives the parameter signals fromthe plurality of mobile stations 21 through the uplink 32 of the radiolink. After the demodulation of the received parameter signals isperformed, the receiver 12 supplies the parameter signals related to themobile stations 21 to the received signal quality determination unit 16,and they are temporarily stored into the memory 13. As described above,each of the parameter signals, received from the mobile stations 21 andstored into the memory 13, indicates the reception power as the receivedsignal quality parameter value.

[0042] In the base station 11 of the present embodiment, thedetermination unit 16 determines a minimum value (Cmin) of the receivedsignal quality parameter values (or the multicast-signal receptionpowers of the mobile stations 21) of the parameter signals, stored inthe memory 13, as the power control value. The minimum value (Cmin) ofthe multicast-signal reception powers of the mobile stations 21 issupplied to one of two inputs of the comparator 14. The referencereception power value (Cref) 15 is always supplied to the other input ofthe comparator 14. In the comparator 14, the minimum value (Cmin) iscompared with the reference reception power value (Cref). The comparator14 supplies a signal, indicating the difference (Cref−Cmin) between thereference reception power value (Cref) and the minimum value (Cmin),from the output of the comparator 14 to the transmission powercontroller 17 as the power control value.

[0043] In the base station 11 of the present embodiment, thetransmission power controller 17 controls the transmission power of themulticast signal 4 based on the power control value (or the receptionpower difference (Cref−Cmin)) indicated by the output signal of thecomparator 14. The above-described control procedure is repeatedlyperformed by the multicast signal transmission system of the presentembodiment during the transmission of the multicast signal 4.

[0044] For example, in the base station 11 of the present embodiment,when Cmin=5 dBm and Cref=0 dBm, the reception power difference(Cref−Cmin) is equal to −5 dBm, and the current transmission power ofthe multicast signal 4 is decreased by the controller 17 by 5 dBm. WhenCmin=−5 dBm and Cref=0 dBm, the reception power difference (Cref−Cmin)is equal to 5 dBm, and the current transmission power of the multicastsignal 4 is increased by the controller 17 by 5 dBm.

[0045] As described above, in the base station 11 of the presentembodiment, the transmitter 3 transmits the multicast signal 4 to eachof the mobile stations 21 through the network, based on the transmissionpower that is properly controlled by the transmission power controller17. Therefore, the multicast signal transmission power control method ofthe present embodiment is effective in reducing the transmission powerof the base station 11 to transmit the multicast signal to the mobilestations 21 without changing the grades of service of the mobilestations 21 which receive the multicast signal and without causing theinterference of the transmission power of the base station 11 to themobile stations 21 which do not receive the multicast signal. In thepresent embodiment, the transmission power of the multicast signal 4 iscontrolled by the transmission power controller 17 such that it issuited to the minimum value of the received signal quality parametervalues received from the respective mobile stations 21.

[0046]FIG. 2 shows a second preferred embodiment of the multicast signaltransmission system in which the transmission power control method ofthe invention is embodied.

[0047] In FIG. 2, the elements that are essentially the same ascorresponding elements in FIG. 1 are designated by the same referencenumerals, and a description thereof will be omitted.

[0048] As described above, according to the transmission power controlmethod of the invention, each of the plurality of mobile stationsmeasures a value of the received signal quality parameter of themulticast signal, which is received from the base station.

[0049] In the present embodiment, each of the mobile stations measures adifference between a measured carrier-to-cochannel interference (C/I)ratio (R) and a reference C/I ratio (Rref) of the received multicastsignal as being the received signal quality parameter value that is thebase to determine a power control value of each mobile station, and thetransmission power of the multicast signal of the base station withrespect to each mobile station is controlled based on the power controlvalue.

[0050] As shown in FIG. 2, in the multicast signal transmission systemof the present embodiment, a base station 31 is provided, and the basestation 31 transmits the multicast signal 4 to a plurality of mobilestations 41 through a mobile radio link. The plurality of mobilestations 41 are provided in the multicast signal transmission system,and each mobile station 41 receives the multicast signal 4 from the basestation 31.

[0051] For the sake of simplicity of description, only one mobilestation 41 is shown in FIG. 2, and the remaining mobile stations 41,which receive the multicast signal 4 from the base station 31, areomitted.

[0052] In the base station 31 shown in FIG. 2, the receiver (RX) 12, thereceived signal quality determination unit (T/P DETERMIN) 16, thetransmitter (TX) 3, and the transmission power controller 17 areprovided. In the determination unit 16, the memory 13 is provided. Asdescribed later, the transmission power controller 17 controls thetransmission power of the multicast signal 4, sent to each of the mobilestations 41, based on the power control value indicated by an outputsignal of the determination unit 16.

[0053] In the mobile station 41 of the present embodiment, the receiver(RX) 6, a carrier-to-cochannel interference (C/I) ratio measurement unit(C/I DETECT) 42, a comparator (COMP) 43, a reference C/I ratio value(Rref) 44, and the transmitter (TX) 23 are provided. In the presentembodiment, the measurement unit 42 is a C/I ratio detector circuit thatmeasures a C/I ratio (R) of the multicast signal 4 received at themobile station 41. The measurement unit 42 supplies a detection signal46, indicating the C/I ratio of the multicast signal 4, to thecomparator 43 as the received signal quality parameter value. In thecomparator 43, the C/I ratio (R) is compared with the reference C/Iratio value (Rref) 44. The comparator 43 outputs a signal, indicating adifference between the reference C/I ratio (Rref) 44 and the measuredC/I ratio (R), to the transmitter 23.

[0054] In the transmitter 23, the carrier wave is modulated inaccordance with the C/I ratio difference, and the transmitter 23transmits a parameter signal, indicating the C/I ratio difference(Rref−R) as the received signal quality parameter value, to the basestation 31 through the uplink 32 of the radio link.

[0055] Similar to the communication system of FIG. 8, in the mobilestation 41 shown in FIG. 2, the multicast signal 4 from the base station31 is received at the receiver (RX) 6. After the demodulation of thereceived multicast signal is performed, the receiver 6 supplies thedemodulated multicast signal to the output terminal 7, and it issupplied from the output terminal 7 to internal units of the mobilestation 41.

[0056] In the multicast signal transmission system of FIG. 2, themeasurement unit 42 measures a C/I ratio of the multicast signal 4received at the mobile station 41. More specifically, the measurementunit 42 measures the C/I ratio of the received multicast signal 4 at asuitable timing that is suited to the structure of the multicast signal4. The timing of the detection of the C/I ratio is the same as that inthe previous embodiment of FIG. 1.

[0057] The measurement unit 42 supplies the signal 46, indicating theC/I ratio of the multicast signal 4 as the received signal qualityparameter value, to one of two inputs of the comparator 43. Thereference C/I ratio (Rref) 44 is always supplied to the other input ofthe comparator 43. The comparator 43 supplies the signal, indicating theC/I ratio difference (Rref−R), to the transmitter 23.

[0058] In the transmitter 23, the carrier wave is modulated inaccordance with the C/I ratio difference (Rref−R). After the modulationis performed, the transmitter 23 transmits the parameter signal,indicating the C/I ratio difference as the received signal qualityparameter value, to the base station 31 through the uplink 32 of theradio link. The uplink 32 may be a channel of the radio link used forrandom access.

[0059] In the base station 31 of the multicast signal transmissionsystem of FIG. 2, the receiver 12 receives the parameter signals fromthe plurality of mobile stations 41 through the uplink 32 of the radiolink. After the demodulation of the received parameter signals isperformed, the receiver 12 supplies the parameter signals related to themobile stations 41 to the received signal quality determination unit 16,and they are temporarily stored into the memory 13. As described above,each of the parameter signals, received from the mobile stations 41 andstored into the memory 13, indicates the C/I ratio difference as thereceived signal quality parameter value.

[0060] In the base station 31 of the present embodiment, thedetermination unit 16 rearranges the received signal quality parametervalues (the C/I ratio differences), which are stored in the memory 13,into a sequence of the parameter values in a predetermined order. Thedetermination unit 16 determines one of the parameter values in therearranged sequence that corresponds to a predetermined ratio of theentire mobile stations 41, as the power control value.

[0061]FIG. 3A, FIG. 3B and FIG. 3C show operations of the determinationunit 16 in the multicast signal transmission system of the presentembodiment.

[0062] In the present embodiment, the received signal quality parametervalues (T/P PARA) of the mobile stations (“A” through “E”) are storedinto the memory 13 in the order of reception of each parameter signal asshown in FIG. 3A. The received signal quality parameter values (or theC/I ratio differences of the mobile stations) are rearranged into asequence of the parameter values in a predetermined order (e.g., in adescending order of each C/I ratio difference) as shown in FIG. 3B. Asshown in FIG. 3C, one of the parameter values in the rearranged sequencethat corresponds to a predetermined ratio (e.g., 80%) of the entiremobile stations “A” through “E” is determined by the determination unit16 of the base station 31 as the power control value.

[0063] The parameter value, which is determined from among the parametervalues of the mobile stations 41 as the power control value, is suppliedfrom the determination unit 16 to the transmission power controller 17as the power control value. In the example of FIG. 3C, the parametervalue that corresponding to the mobile station “B” (80% of the fivemobile stations) is +8 dB, and the determination unit 16 outputs thesignal, indicating +8 dB as the power control value, to the transmissionpower controller 17.

[0064] In the base station 31 of the present embodiment, thetransmission power controller 17 controls the transmission power of themulticast signal 4 based on the power control value (or the determinedparameter value) indicated by the output signal of the determinationunit 16. The above-described control procedure is repeatedly performedby the multicast signal transmission system of the present embodimentduring the transmission of the multicast signal 4.

[0065] For example, in the base station 31 of the present embodiment,when the power control value is equal to +8 dB, the current transmissionpower of the multicast signal 4 is changed to +8 dB by the transmissionpower controller 17.

[0066] As described above, in the base station 31 of the presentembodiment, the transmitter 3 transmits the multicast signal 4 to eachof the mobile stations 41 through the network, based on the transmissionpower that is properly controlled by the transmission power controller17. Therefore, the multicast signal transmission power control method ofthe present embodiment is effective in reducing the transmission powerof the base station 31 to transmit the multicast signal to the mobilestations 41 without changing the grades of service of the mobilestations 41 which receive the multicast signal and without causing theinterference of the transmission power of the base station 31 to themobile stations 41 which do not receive the multicast signal. In thepresent embodiment, the transmission power of the multicast signal 4 iscontrolled by the transmission power controller 17 such that it issuited to one of the parameter values in the rearranged sequence thatcorresponds to a predetermined ratio of the entire mobile stations 41.

[0067]FIG. 4 shows a third preferred embodiment of the multicast signaltransmission system in which the transmission power control method ofthe invention is embodied.

[0068] In FIG. 4, the elements that are essentially the same ascorresponding elements in FIG. 1 are designated by the same referencenumerals, and a description thereof will be omitted.

[0069] In the present embodiment, each of the mobile stations measuresan error ratio (E/R) of the received multicast signal to a given bitpattern as being the received signal quality parameter value that is thebase to determine a power control value of each mobile station, and thetransmission power of the multicast signal of the base station withrespect to each mobile station is controlled based on the power controlvalue.

[0070] As shown in FIG. 4, in the multicast signal transmission systemof the present embodiment, a base station 51 is provided, and the basestation 51 transmits the multicast signal 4 to a plurality of mobilestations 61 through the mobile radio link. The plurality of mobilestations 61 are provided in the multicast signal transmission system,and each mobile station 61 receives the multicast signal 4 from the basestation 51.

[0071] For the sake of simplicity of description, only one mobilestation 61 is shown in FIG. 4, and the remaining mobile stations 61,which receive the multicast signal 4 from the base station 51, areomitted.

[0072] In the base station 51 shown in FIG. 4, the receiver (RX) 12, thereceived signal quality determination unit (T/P DETERMIN) 16, thetransmitter (TX) 3, and the transmission power controller 17 areprovided. In the determination unit 16, the memory 13 and apower-to-error ratio map (PWR-TO-E/R MAP) 52 are provided. Thepower-to-error ratio map 52 provides a relationship between thetransmission power and the error ratio for the determination unit 16. Asdescribed later, the determination unit 16 determines a power controlvalue of each mobile station based on the output of the map 52 inresponse to the parameter signal received from the mobile station. Thetransmission power controller 17 controls the transmission power of themulticast signal 4, sent to each of the mobile stations 61, based on thepower control value indicated by the output signal of the determinationunit 16.

[0073] In the base station 51, the original message of the multicastsignal 4 is supplied from the input terminal 2, and it is input to thetransmitter (TX) 3. In the transmitter 3, the carrier wave is modulatedin accordance with the original message from the input terminal 2, sothat the multicast signal 4 is produced. In the transmission powercontroller 17, the transmission power of the multicast signal 4, sent toeach of the mobile stations 61, is controlled based on the power controlvalue indicated by the output signal of the determination unit 16. Afterthe modulation is performed, the transmitter 3 transmits the multicastsignal 4 to each of the mobile stations 61 through the network, based onthe controlled transmission power of the transmission power controller17.

[0074] In the mobile station 61, the receiver (RX) 6, an error ratiomeasurement unit (E/R DETECT) 62 and the transmitter (TX) 23 areprovided. In the present embodiment, the measurement unit 62 is an errorratio detector circuit that measures an error ratio of the receivedmulticast signal 4 to the given bit pattern. The measurement unit 62supplies a detection signal, indicating the error ratio of the multicastsignal 4 as the received signal quality parameter value, to thetransmitter 23. In the transmitter 23, the carrier wave is modulated inaccordance with the error ratio, and the transmitter 23 transmits aparameter signal, indicating the error ratio as the received signalquality parameter value of the mobile station 61, to the base station 51through the uplink 32 of the radio link.

[0075] Similar to the communication system of FIG. 8, in the mobilestation 61 shown in FIG. 4, the multicast signal 4 from the base station51 is received at the receiver (RX) 6. After the demodulation of thereceived multicast signal is performed, the receiver 6 supplies thedemodulated multicast signal to the output terminal 7, and it issupplied from the output terminal 7 to internal units of the mobilestation 61.

[0076] In the base station 51 of the multicast signal transmissionsystem of FIG. 4, the receiver 12 receives the parameter signals fromthe plurality of mobile stations 61 through the uplink 32 of the radiolink. After the demodulation of the received parameter signals isperformed, the receiver 12 supplies the parameter signals related to themobile stations 61 to the received signal quality determination unit 16,and they are temporarily stored into the memory 13. As described above,each of the parameter signals, which are demodulated by the receiver 12and stored into the memory 13, indicates the error ratio as the receivedsignal quality parameter value of the related mobile station 61.

[0077] In the base station 51 of the present embodiment, thedetermination unit 16 selects a maximum value of the received signalquality parameter values (or the multicast-signal error ratios of themobile stations 61) of the parameter signals, stored in the memory 13.The maximum value of the multicast-signal error ratios of the mobilestations 61 is supplied to the power-to-error ratio map 52. In response,the determination unit 16 receives a power value of the map 52corresponding to the selected maximum error ratio, and determines thecorresponding power value as being the power control value. Thedetermination unit 16 outputs a signal, indicating the correspondingpower value, to the transmission power controller 17 as the powercontrol value.

[0078] In the base station 51 of the present embodiment, thetransmission power controller 17 controls the transmission power of themulticast signal 4 based on the power control value (or thecorresponding power value of the map 52 for the selected maximum errorratio) indicated by the output signal of the determination unit 16. Theabove-described control procedure is repeatedly performed by themulticast signal transmission system of the present embodiment duringthe transmission of the multicast signal 4.

[0079] As described above, in the base station 51 of the presentembodiment, the transmitter 3 transmits the multicast signal 4 to eachof the mobile stations 61 through the network, based on the transmissionpower that is properly controlled by the transmission power controller17. Therefore, the multicast signal transmission power control method ofthe present embodiment is effective in reducing the transmission powerof the base station 51 to transmit the multicast signal to the mobilestations 61 without changing the grades of service of the mobilestations 61 which receive the multicast signal and without causing theinterference of the transmission power of the base station 51 to themobile stations 61 which do not receive the multicast signal. In thepresent embodiment, the transmission power of the multicast signal 4 iscontrolled by the transmission power controller 17 such that it issuited to the maximum error ratio among the multicast-signal errorratios of the mobile stations 61.

[0080] In the multicast signal transmission system of theabove-described embodiment, each of the mobile stations measures anerror ratio of the received multicast signal to a given bit pattern asbeing the received signal quality parameter value that is the base todetermine a power control value of each mobile station. The transmissionpower control method of the invention is not limited to this embodiment.Alternatively, the multicast signal transmission system of the presentinvention may be configured such that each of the mobile stationsmeasures one of a bit error ratio, a packet error ratio and a slot errorratio of the received multicast signal as being the value of thereceived signal quality parameter. Alternatively, the multicast signaltransmission system of the present invention may be configured such thateach of the mobile stations measures one of an error-correction-bitnumber and a maximum likelihood value, obtained by decoding of thereceived multicast signal, as being the value of the received signalquality parameter.

[0081]FIG. 5 shows a fourth preferred embodiment of the multicast signaltransmission system in which the transmission power control method ofthe invention is embodied.

[0082] In FIG. 5, the elements that are essentially the same ascorresponding elements in FIG. 1 are designated by the same referencenumerals, and a description thereof will be omitted.

[0083] Suppose that, in the present embodiment, the multicast signal,which is transmitted from the base station (BS) to the plurality ofmobile stations (MS) through the radio link, is provided in the form ofblocks, and the transmission of the multicast signal is performed on ablock basis. Suppose that cyclic redundancy check (CRC) codes for errordetection of a received multicast signal are added to each of themulticast signal blocks.

[0084] In the present embodiment, when an error in demodulation of areceived multicast signal is detected, each of the mobile stationstransmits an automatic repeat request (ARQ) signal to the base station,and a transmission power of the multicast signal of the base stationwith respect to each mobile station is controlled based on the ARQsignal received from the mobile station.

[0085] As shown in FIG. 5, in the multicast signal transmission systemof the present embodiment, a base station 71 is provided, and the basestation 71 transmits the multicast signal 4 to a plurality of mobilestations 81 through the mobile radio link. The plurality of mobilestations 81 are provided in the multicast signal transmission system,and each mobile station 81 receives the multicast signal 4 from the basestation 71.

[0086] For the sake of simplicity of description, only one mobilestation 81 is shown in FIG. 5, and the remaining mobile stations 81,which receive the multicast signal 4 from the base station 71, areomitted.

[0087] In the base station 71 shown in FIG. 5, the receiver (RX) 12, anARQ signal counter unit (PC) 73, the transmitter (TX) 3, and thetransmission power controller 17 are provided. The signal counter unit73 counts the number of ARQ signals received from the receiver 12. Asdescribed later, the transmission power controller 17 controls atransmission power of the multicast signal 4, sent to each of the mobilestations 81, based on a result of the ARQ-signal detection indicated byan output signal of the signal counter unit 73.

[0088] In the base station 71, the original message of the multicastsignal 4 is supplied from the input terminal 2, and it is input to thetransmitter (TX) 3. In the transmitter 3, the carrier wave is modulatedin accordance with the original message from the input terminal 2, sothat the multicast signal 4 is produced. In the transmission powercontroller 17, the transmission power of the multicast signal 4, sent toeach of the mobile stations 81, is controlled based on the power controlvalue indicated by the output signal of the signal counter unit 73.After the modulation is performed, the transmitter 3 transmits themulticast signal 4 to each of the mobile stations 81 through thenetwork, based on the controlled transmission power of the transmissionpower controller 17.

[0089] In the mobile station 81, the receiver (RX) 6, an errordetection/automatic repeat request (ED/ARQ) unit 82 and the transmitter(TX) 23 are provided. In the present embodiment, the ED/ARQ unit 82detects whether an error in demodulation of the received multicastsignal 4 takes place. The detection of such error in the demodulation ofthe received multicast signal 4 is performed by the ED/ARQ unit 82 on ablock basis with respect to the received multicast signal 4. When anerror in the demodulation of a certain block of the received multicastsignal is detected, the ED/ARQ unit 82 supplies an ARQ signal 84,indicating the occurrence of the error, to the transmitter 23. After themodulation of the ARQ signal 84 is performed, the transmitter 23transmits the ARQ signal 84 (e.g., a negative-acknowledge (NAK) signal),indicating a request for retransmission of the multicast signal to themobile station 81, to the base station 71 through the uplink 32 of theradio link.

[0090] On the other hand, when no error in the demodulation of one blockof the received multicast signal takes place, the transmitter 23transmits an acknowledge (ACK) signal to the base station 71 through theuplink 32, which causes the mobile station 81 to be set in a waitingcondition for receiving a following block of the multicast signal 4.

[0091] Similar to the communication system of FIG. 8, in the mobilestation 81 shown in FIG. 5, the multicast signal 4 from the base station71 is received at the receiver (RX) 6. After the demodulation of thereceived multicast signal is performed, the receiver 6 supplies thedemodulated multicast signal to the output terminal 7, and it issupplied from the output terminal 7 to internal units of the mobilestation 81.

[0092] In the base station 71 of the multicast signal transmissionsystem of FIG. 5, the receiver 12 receives the ARQ signals from theplurality of mobile stations 81 through the uplink 32 of the radio link.After the demodulation of the received parameter signals is performed,the receiver 12 supplies the ARQ signals of the mobile stations 81 tothe signal counter unit 73. At the same time, the ARQ signals of themobile stations 81 are supplied from an output terminal 75 into internalunits of the base station 71. When the ARQ signal of one of the mobilestations 81 is received, the base station 71 transmits again theerror-detected block of the multicast signal to the mobile station 81(this is not shown in FIG. 5).

[0093] In the base station 71 of the present embodiment, the signalcounter unit 73 temporarily stores the ARQ signals sent by the receiver12, and counts the number of the stored ARQ signals at intervals of apredetermined time. Suppose that, in the present embodiment, thepredetermined time for counting the number of the stored ARQ signals istwice the round-trip delay of the multicast signal blocks.

[0094] In the base station 71 of the multicast signal transmissionsystem of FIG. 5, when the number of the stored ARQ signals, counted bythe signal counter unit 73, is larger than zero, the signal counter unit73 supplies a first power control signal to the transmission powercontroller 17 so as to increase the current transmission power of themulticast signal 4. On the other hand, when the number of the stored ARQsignals, counted by the signal counter unit 73, is equal to zero, thesignal counter unit 73 supplies a second power control signal to thetransmission power controller 17 so as to decrease the currenttransmission power of the multicast signal 4.

[0095] In the present embodiment, the amount of the transmission powerthat is changed (increased or decreased) from the current transmissionpower to a new transmission power is predetermined.

[0096] In the base station 71 of the present embodiment, thetransmission power controller 17 controls the transmission power of themulticast signal 4 based on the power control signal sent by the signalcounter unit 73. The above-described control procedure is repeatedlyperformed by the multicast signal transmission system of the presentembodiment during the transmission of the multicast signal 4.

[0097]FIG. 6 is a timing chart for explaining operations of themulticast signal transmission system of the fourth preferred embodiment.

[0098] In FIG. 6, (A) indicates changes of the transmission power of themulticast signal at the base station (BS) with respect to the elapsedtime, (B) indicates a sequence of the multicast signal blockstransmitted by the base station (BS) with respect to the elapsed time,and (C) indicates a sequence of the multicast signal blocks received bythe mobile station (MS) with respect to the elapsed time. For the sakeof simplicity of description, only one mobile station 81, as shown inFIG. 5, is considered.

[0099] As indicated by (B) in FIG. 6, the sequence of the multicastsignal blocks is transmitted by the base station 71 to the mobilestation 81. Suppose that the cyclic redundancy check (CRC) codes forerror detection of a received multicast signal are added to each of themulticast signal blocks. Suppose that the round-trip delay is equal to atransmission time of two multicast signal blocks. In other words, it issupposed that the timing the base station 71 receives an ARQ signalrelated to the multicast signal block “1” from the mobile station 81precedes the start of the transmission of the multicast signal block“3”. Further, suppose that the timing the base station 71 controls thetransmission power of the multicast signal is twice the round-trip delayof the multicast signal blocks, namely, it is equal to a transmissiontime of four multicast signal blocks.

[0100] As shown in FIG. 6, no ARQ signal related to the blocks “1” and“2” is detected by the base station 71 prior to the start of thetransmission of the blocks “3” and “4”, and the first transmission power“P1” is decreased to the second transmission power “P2” at the start ofthe transmission of the block “5”. The transmission power of the blocks“1” to “4” is maintained at “P1”.

[0101] After the start of the transmission of the block “5”, no ARQsignal related to the blocks “5” and “6” is detected by the base station71 prior to the start of the transmission of the blocks “7” and “8”, andthe second transmission power “P2” is decreased to the thirdtransmission power “P3” at the start of the transmission of the block“9”. The transmission power of the multicast signal blocks “5” to “8” ismaintained at “P2”.

[0102] After the start of the transmission of the block “9”, the ARQsignals related to the blocks “9” and “10” are detected by the basestation 71 prior to the start of the transmission of the blocks “11” and“12”, and the third transmission power “P3” is increased to the secondtransmission power “P2” at the start of the transmission of the block“11”. The transmission power control procedures that are the same asthose described above are continuously performed.

[0103] As described above, in the base station 71 of the presentembodiment, the transmitter 3 transmits the multicast signal 4 to eachof the mobile stations 81 through the network, based on the transmissionpower that is properly controlled by the transmission power controller17. Therefore, the multicast signal transmission power control method ofthe present embodiment is effective in reducing the transmission powerof the base station 71 to transmit the multicast signal to the mobilestations 81 without changing the grades of service of the mobilestations 81 which receive the multicast signal and without causing theinterference of the transmission power of the base station 11 to themobile stations 81 which do not receive the multicast signal.

[0104] In the above-described embodiment of FIG. 6, the base stationcontrols the transmission power of the multicast signal based on whetherthe ARQ signal (NAK) from the mobile station is received at the basestation or not. In the present embodiment, the signal counter unit 73,which counts the number of ARQ signals received from the mobile stationsat the receiver 12, is not necessarily needed in the base station 71.Hence, in the present embodiment, the signal counter unit 73 in the basestation 71 may be omitted if a measurement unit, which detects whetherthe ARQ signal (NAK) from the mobile station is received at the basestation, is provided in the base station 71.

[0105] Alternatively, in the multicast signal transmission system of thepresent invention, another signal counter unit, which detects whether aratio of the number of the received ARQ signals to the total number ofthe mobile stations exceeds a predetermined ratio, may be provided inthe base station. In such alternative embodiment, the base stationcontrols the transmission power of the multicast signal depending onwhether the ratio of the number of the received ARQ signals to the totalnumber of the mobile stations exceeds the predetermined ratio.

[0106]FIG. 7 shows a fifth preferred embodiment of the multicast signaltransmission system in which the transmission power control method ofthe invention is embodied.

[0107] In FIG. 7, the elements that are essentially the same ascorresponding elements in FIG. 1 are designated by the same referencenumerals, and a description thereof will be omitted.

[0108] In the present embodiment, when an error in demodulation of thereceived multicast signal is detected, each of the mobile stationstransmits an ARQ signal to the base station, and the transmission powerof the multicast signal of the base station with respect to each mobilestation is controlled depending on whether the ratio of the received ARQsignals to the total number of the mobile stations exceeds apredetermined ratio. Further, in the present embodiment, when no errorin demodulation of the received multicast signal is detected, each ofthe mobile stations transmits an acknowledge (ACK) signal to the basestation, in order to confirm that the multicast signal from the basestation is received at the mobile station without error.

[0109] As shown in FIG. 7, in the multicast signal transmission systemof the present embodiment, a base station 91 is provided, and the basestation 91 transmits the multicast signal 4 to a plurality of mobilestations 101 through the mobile radio link. The plurality of mobilestations 101 are provided in the multicast signal transmission system,and each mobile station 101 receives the multicast signal 4 from thebase station 91.

[0110] For the sake of simplicity of description, only one mobilestation 101 is shown in FIG. 7, and the remaining mobile stations 101,which receive the multicast signal 4 from the base station 91, areomitted.

[0111] In the base station 91 shown in FIG. 7, the receiver (RX) 12, anARQ signal counter unit (PC) 93, the transmitter (TX) 3, and thetransmission power controller 17 are provided. The signal counter unit93 counts the number of ARQ signals received from the mobile stations101 and the number of ACK signals received from the mobile stations 101.The signal counter unit 93 detects whether the ratio of the number ofthe received ARQ signals to the total number of the mobile stations 101(the sum of the number of the received ARQ signals and the number of thereceived ACK signals) exceeds a predetermined ratio. As described later,the transmission power controller 17 controls the transmission power ofthe multicast signal 4, sent to each of the mobile stations 101, basedon a result of the ARQ ratio detection indicated by an output signal ofthe signal counter unit 93.

[0112] In the base station 91, the original message of the multicastsignal 4 is supplied from the input terminal 2, and it is input to thetransmitter (TX) 3. In the transmitter 3, the carrier wave is modulatedin accordance with the original message from the input terminal 2, sothat the multicast signal 4 is produced. In the transmission powercontroller 17, the transmission power of the multicast signal 4, sent toeach of the mobile stations 101, is controlled based on the result ofthe ARQ ratio detection indicated by the output signal of the signalcounter unit 93. After the modulation is performed, the transmitter 3transmits the multicast signal 4 to each of the mobile stations 101through the network, based on the controlled transmission power of thetransmission power controller 17.

[0113] In the mobile station 101, the receiver (RX) 6, an errordetection/automatic repeat request (ED/ARQ) unit 102 and the transmitter(TX) 23 are provided. In the present embodiment, the ED/ARQ unit 102detects whether an error in demodulation of the received multicastsignal 4 takes place. The detection of such error in the demodulation ofthe received multicast signal 4 is performed by the ED/ARQ unit 102 on ablock basis with respect to the received multicast signal 4. When anerror in the demodulation of a certain block of the received multicastsignal is detected, the ED/ARQ unit 102 supplies an ARQ signal 104(e.g., a negative-acknowledge (NAK) signal), indicating the presence ofa demodulation error and a request for retransmission of theerror-detected block of the multicast signal to the mobile station 101,to the transmitter 23. When no error in the demodulation of a certainblock of the received multicast signal is detected, the ED/ARQ unit 102supplies an ACK signal 105, indicating the absence of a demodulationerror, to the transmitter 23. After the modulation of the ARQ signal 104or the ACK signal 105 is performed, the transmitter 23 transmits eitherthe ARQ signal 104 or the ACK signal 105, to the base station 91 throughthe uplink 32 of the radio link.

[0114] Similar to the communication system of FIG. 8, in the mobilestation 101 shown in FIG. 7, the multicast signal 4 from the basestation 91 is received at the receiver (RX) 6. After the demodulation ofthe received multicast signal is performed, the receiver 6 supplies thedemodulated multicast signal to the output terminal 7, and it issupplied from the output terminal 7 to internal units of the mobilestation 101.

[0115] In the base station 91 of the multicast signal transmissionsystem of FIG. 7, the receiver 12 receives the ARQ signals and the ACKsignals from the plurality of mobile stations 101 through the uplink 32of the radio link. After the demodulation of the received ARQ/ACKsignals is performed, the receiver 12 supplies the ARQ/ACK signals ofthe mobile stations 101 to the signal counter unit 93. At the same time,the ARQ/ACK signals of the mobile stations 101 are supplied from anoutput terminal 95 into internal units of the base station 91. When theARQ signal of one of the mobile stations 101 is received, the basestation 91 transmits again the error-detected block of the multicastsignal to the mobile station 101 (this is not shown in FIG. 7).

[0116] In the base station 71 of the present embodiment, the signalcounter unit 73 temporarily stores the ARQ signals sent by the receiver12, and counts the number of the stored ARQ signals at intervals of apredetermined time. At the same time, the signal counter unit 73temporarily stores the ACK signals sent by the receiver 12, and countsthe number of the stored ACK signals at intervals of the predeterminedtime.

[0117] In the base station 91 of the multicast signal transmissionsystem of FIG. 7, the signal counter unit 73 calculates a ratio of thenumber of the received ARQ signals to the sum of the number of thereceived ARQ signals and the number of the received ACK signals. The sumof the ARQ-signal number and the ACK-signal number is equal to the totalnumber of the mobile stations 101. Then, when the ratio of theARQ-signal number to the total number of the mobile stations 101 exceedsthe predetermined ratio, the signal counter unit 93 outputs a firstpower control signal to the transmission power controller 17 so as toincrease the current transmission power of the multicast signal 4. Onthe other hand, when the ratio of the ARQ-signal number to the totalnumber of the mobile stations 101 does not exceed the predeterminedratio, the signal counter unit 93 outputs a second power control signalto the transmission power controller 17 so as to decrease the currenttransmission power of the multicast signal 4.

[0118] In the present embodiment, the amount of the transmission powerthat is changed (increased or decreased) from the current transmissionpower to a new transmission power is predetermined.

[0119] In the base station 91 of the present embodiment, thetransmission power controller 17 controls the transmission power of themulticast signal 4 based on the power control signal sent by the signalcounter unit 93. The above-described control procedure is repeatedlyperformed by the multicast signal transmission system of the presentembodiment during the transmission of the multicast signal 4.

[0120] In the above-described embodiment, the signal counter unit 73calculates a ratio of the number of the received ARQ signals to the sumof the number of the received ARQ signals and the number of the receivedACK signals. Alternatively, the signal counter unit 73 may calculate aratio of the number of the received ACK signals to the sum of the numberof the received ARQ signals and the number of the received ACK signals.In such alternative embodiment, when the ratio of the ACK-signal numberto the total number of the mobile stations 101 exceeds a predeterminedratio, the signal counter unit 93 outputs a first power control signalto the transmission power controller 17 so as to decrease the currenttransmission power of the multicast signal 4. On the other hand, whenthe ratio of the ACK-signal number to the total number of the mobilestations 101 does not exceed the predetermined ratio, the signal counterunit 93 outputs a second power control signal to the transmission powercontroller 17 so as to increase the current transmission power of themulticast signal 4.

[0121] The present invention is not limited to the above-describedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

[0122] Further, the present invention is based on Japanese priorityapplication No.2000-105231, filed on Apr. 6, 2000, and Japanese priorityapplication No.2000-105232, filed on Apr. 6, 2000, the entire contentsof which are hereby incorporated by reference.

What is claimed is
 1. A method of controlling a transmission power of amulticast signal that is transmitted from a base station to a pluralityof mobile stations through a radio link, comprising the steps of:measuring a value of a received signal quality parameter of a multicastsignal received at the mobile stations; transmitting a parameter signal,indicating the received signal quality parameter value, from the mobilestations to the base station through the radio link; receiving theparameter signals from the mobile stations at the base station throughthe radio link; determining a power control value of each of the mobilestations based on the received signal quality parameter values of theparameter signals received at the base station; and controlling thetransmission power of the multicast signal, sent to each of the mobilestations, based on the determined power control value.
 2. The methodaccording to claim 1 , wherein, in said determining step, a minimumvalue of the received signal quality parameter values of the receivedparameter signals is determined as being the power control value.
 3. Themethod according to claim 1 , wherein, in said determining step, thereceived signal quality parameter values are rearranged into a sequenceof the parameter values in a predetermined order, and one of theparameter values in the rearranged sequence that corresponds to apredetermined ratio of the entire mobile stations is determined as beingthe power control value.
 4. The method according to claim 1 , wherein areception power of the received multicast signal is measured as beingthe value of the received signal quality parameter in said measuringstep.
 5. The method according to claim 1 , wherein, in said measuringstep, a carrier-to-cochannel interference C/I ratio of the receivedmulticast signal is measured, and a difference between the measured C/Iratio and a reference C/I ratio is measured as being the value of thereceived signal quality parameter.
 6. The method according to claim 1 ,wherein one of a bit error ratio, a packet error ratio and a slot errorratio of the received multicast signal is measured as being the value ofthe received signal quality parameter in said measuring step.
 7. Themethod according to claim 1 , wherein one of an error-correction-bitnumber and a maximum likelihood value, obtained by decoding of thereceived multicast signal, is measured as being the value of thereceived signal quality parameter in said measuring step.
 8. A basestation which controls a transmission power of a multicast signal thatis transmitted to a plurality of mobile stations through a radio link,comprising: a receiver receiving parameter signals from the mobilestations through the radio link, each parameter signal indicating avalue of a received signal quality parameter of the multicast signalreceived at one of the mobile station; a determination unit determininga power control value of each of the mobile stations based on thereceived signal quality parameter values of the parameter signalsreceived by the receiver; and a transmission power controllercontrolling the transmission power of the multicast signal, sent to eachof the mobile stations, based on the determined power control value. 9.The base station according to claim 8 , wherein the determination unitdetermines a minimum value of the received signal quality parametervalues of the received parameter signals as being the power controlvalue.
 10. The base station according to claim 8 , wherein thedetermination unit rearranges the received signal quality parametervalues into a sequence of the parameter values having a predeterminedorder, and determines one of the parameter values in the rearrangedsequence that corresponds to a predetermined ratio of the entire mobilestations as being the power control value.
 11. The base stationaccording to claim 8 , wherein each of the mobile stations measures areception power of the received multicast signal as being the value ofthe received signal quality parameter.
 12. The base station according toclaim 8 , wherein each of the mobile stations measures acarrier-to-cochannel interference C/I ratio of the received multicastsignal, and measures a difference between the measured C/I ratio and areference C/I ratio as being the value of the received signal qualityparameter.
 13. The base station according to claim 8 , wherein each ofthe mobile stations measures one of a bit error ratio, a packet errorratio and a slot error ratio of the received multicast signal as beingthe value of the received signal quality parameter.
 14. The base stationaccording to claim 8 , wherein each of the mobile stations measures oneof an error-correction-bit number and a maximum likelihood value,obtained by decoding of the received multicast signal, as being thevalue of the received signal quality parameter.
 15. A method ofcontrolling a transmission power of a multicast signal that istransmitted from a base station to a plurality of mobile stationsthrough a radio link, comprising the steps of: transmitting themulticast signal to the mobile stations through the radio link;receiving an automatic repeat request ARQ signal from each of the mobilestations at the base station through the radio link, wherein each of themobile stations transmits the ARQ signal to the base station when anerror in demodulation of a received multicast signal occurs; detectingwhether at least one of a plurality of ARQ signals from the mobilestations is received at the base station; outputting a power controlsignal indicating a result of the ARQ-signal detection; and controllingthe transmission power of the multicast signal, sent to each of themobile stations, based on the ARQ-signal detection result indicated bythe power control signal.
 16. The method according to claim 15 ,wherein, in said controlling step, the transmission power of themulticast signal is increased when at least one of the ARQ signals isreceived at the base station, and the transmission power of themulticast signal is decreased when none of the ARQ signals is receivedat the base station.
 17. The method according to claim 15 , wherein, insaid detecting step, it is detected whether a ratio of the number of thereceived ARQ signals to the number of the mobile stations exceeds apredetermined ratio, and, in said controlling step, the transmissionpower of the multicast signal is increased when the ratio of theARQ-signal number exceeds the predetermined ratio, and the transmissionpower of the multicast signal is decreased when the ratio of theARQ-signal number does not exceed the predetermined ratio.
 18. A basestation which controls a transmission power of a multicast signal thatis transmitted to a plurality of mobile stations through a radio link,comprising: a transmitter transmitting the multicast signal to themobile stations through the radio link; a receiver receiving anautomatic repeat request ARQ signal from each of the mobile stationsthrough the radio link, wherein each of the mobile stations transmitsthe ARQ signal to the base station when an error in demodulation of areceived multicast signal occurs; a signal counter unit detectingwhether at least one of a plurality of ARQ signals from the mobilestations is received by the receiver, and outputting a power controlsignal indicating a result of the ARQ-signal detection; and atransmission power controller controlling the transmission power of themulticast signal, sent to each of the mobile stations, based on theARQ-signal detection result indicated by the power control signal of thesignal counter unit.
 19. The base station according to claim 18 ,wherein the transmission power controller is configured to increase thetransmission power of the multicast signal when at least one of the ARQsignals is received by the receiver, and to decrease the transmissionpower of the multicast signal when none of the ARQ signals is receivedby the receiver.
 20. The base station according to claim 18 , whereinthe signal counter unit is configured to detect whether a ratio of thenumber of the received ARQ signals to the number of the mobile stationsexceeds a predetermined ratio, and wherein the transmission powercontroller is configured to increase the transmission power of themulticast signal when the ratio of the ARQ-signal number exceeds thepredetermined ratio, and to decrease the transmission power of themulticast signal when the ratio of the ARQ-signal number does not exceedthe predetermined ratio.